1. Field of the Invention
The present invention relates to an MVA (Multi-domain Vertical Alignment) type liquid crystal display device in which one pixel includes plural alignment areas different from each other in the alignment direction of liquid crystal molecules, and particularly to a liquid crystal display device in which a pixel area is divided into plural sub-pixels.
2. Description of the Related Art
A liquid crystal display device is thin and lightweight as compared with a CRT (Cathode Ray Tube), and has merits that it can be driven by low voltage and its electric power consumption is small. Thus, the liquid crystal display device is used for various electronic devices such as a notebook PC (personal computer), a PDA (Personal Digital Assistant) and a cellular phone. Especially, an active matrix type liquid crystal display device in which a TFT (Thin Film Transistor) is provided as a switching element for each pixel has high drive capability. Since the active matrix type liquid crystal display device has excellent display characteristics comparable to a CRT, it comes to be used for a use for which a CRT is conventionally used, such as a desktop PC or a television receiver.
FIG. 15 shows a rough sectional structure of a conventional liquid crystal display device. As shown in FIG. 15, the liquid crystal display device includes a liquid crystal display panel 101. The liquid crystal display panel 101 includes a TFT substrate 102 on which a TFT and a pixel electrode are formed for each pixel, an opposite substrate 104 on which a color filter (CF) and a common electrode are formed, and a liquid crystal 106 sealed between both the substrates 102 and 104. Since a connection terminal is provided, the TFT substrate 102 is formed to be larger than the opposite substrate 104. Both the substrates 102 and 104 are bonded to each other through a sealing material 152 applied to an outer peripheral part. A cell gap between both the substrates 102 and 104 is kept by, for example, spherical spacers 146. Besides, polarizing plates 187 and 186 are disposed at both the outsides of the liquid crystal display panel 101. A backlight unit (not shown) is disposed below the polarizing plate 187 in the drawing.
Conventionally, a TN (Twisted Nematic) mode liquid crystal display device is widely used which includes a horizontally aligned liquid crystal having positive dielectric anisotropy and in which a liquid crystal molecule is twist-aligned. However, the TN mode liquid crystal display device has defects that a viewing angle characteristic is poor and when a screen is viewed from an oblique direction, the contrast and hue are remarkably changed. Thus, a VA (Vertically Aligned) mode liquid crystal display device having an excellent viewing angle characteristic and an MVA type liquid crystal display device have been developed and have been put to practical use.
FIGS. 16A and 16B schematically show sectional structures of an MVA type liquid crystal display device. A vertically aligned liquid crystal 106 having negative dielectric anisotropy is sealed between a TFT substrate 102 and an opposite substrate 104. A bank-shaped linear projection 143 as an alignment regulating structure to regulate the alignment of the liquid crystal 106 is formed on a pixel electrode 116 of the TFT substrate 102. A vertically aligned film 150 made of, for example, polyimide is formed on the pixel electrode 116 and the linear projection 143.
A bank-shaped linear projections 142 as an alignment regulating structures is formed on a common electrode 141 of the opposite substrate 104. The linear projection 142 is extended in parallel to the linear projection 143 on the TFT substrate 102 side, and is arranged to be shifted from the linear projection 143 by a half pitch. A vertically aligned film 151 made of, for example, polyimide is formed on the common electrode 141 and the linear projection 142.
In the MVA type liquid crystal display device, in a state where a voltage is not applied between the pixel electrode 116 and the common electrode 141, as shown in FIG. 16A, almost all liquid crystal molecules 108 are aligned almost perpendicularly to the substrate surface. However, the liquid crystal molecules 108 in the vicinities of the linear projections 142 and 143 are aligned almost perpendicularly to the inclined surfaces of the linear projections 142 and 143.
When a specified voltage is applied between the pixel electrode 116 and the common electrode 141, the liquid crystal molecules 108 are inclined with respect to the substrate surface by the influence of an electric field. In this case, as shown in FIG. 16B, the inclined directions of the liquid crystal molecule 108 are different at both sides of each of the linear projections 142 and 143. By this, the so-called alignment division (multi-domain) is realized.
As shown in FIG. 16B, in the MVA type liquid crystal display device, since the inclined directions of the liquid crystal molecules 108 at the time when the voltage is applied are different at both sides of each of the linear projections 142 and 143, the leakage of light in an oblique direction is suppressed, and an excellent viewing angle characteristic is obtained.
In the above example, although the description has been given to the case where the alignment regulating structures are the linear projections 142 and 143, a slit obtained by partially removing an electrode or a recess (groove) of a substrate surface may be used as an alignment regulating structure. Besides, in FIGS. 16A and 16B, although the description has been given to the example in which the alignment regulating structures are provided on both the TFT substrate 102 and the opposite substrate 104, the alignment regulating structure may be formed only on one of the TFT substrate 102 and the opposite substrate 104.
FIG. 17 shows an example in which a slit 145 as an alignment regulating structure is formed only in a pixel electrode 116 on a TFT substrate 102 side. The electric field is distorted in the vicinity of the slit 145, and the electric line of force extends in an oblique direction with respect to the substrate surface, and therefore, the inclined directions of liquid crystal molecules 108 are different at both sides of the slit 145. By this, the alignment division can be realized and the viewing angle characteristic is improved.
FIG. 18 shows a structure of one pixel of an MVA type liquid crystal display device in which a slit 145 is formed on a TFT substrate 102 side, and a linear projection 142 is formed on an opposite substrate 104 side. FIG. 19 shows a sectional structure of the TFT substrate 102 cut along line X-X of FIG. 18. As shown in FIG. 18 and FIG. 19, plural gate bus lines 112 extending in the horizontal direction in the drawing and plural drain bus lines 114 extending in the vertical direction in the drawing are respectively disposed at specified pitches on the TFT substrate 102. Rectangular pixel areas are defined by the gate bus liens 112 and the drain bus lines 114. Besides, on the TFT substrate 102, a storage capacitor bus line 118 is formed to be arranged in parallel with the gate bus line 112 and to cross the center part of each of the pixel areas. An insulating film 130 is formed between the drain bus line 114 and the gate bus line 112 or the storage capacitor bus line 118. The gate bus line 112 and the drain bus line 114, and the storage capacitor bus line 118 and the drain bus line 114 are electrically isolated by the insulating film 130.
A TFT 120, a pixel electrode 116 and a storage capacitor electrode 119 are formed for each of the pixel areas. The TFT 120 uses a part of the gate bus line 112 as its gate electrode. Besides, a drain electrode 121 of the TFT 120 is connected to the drain bus line 114, and a source electrode 122 is formed at a position opposite to the drain electrode 121 across the gate bus line 112. Further, the storage capacitor electrode 119 is formed at a position opposite to the storage capacitor bus line 118 across the insulating film 130.
The storage capacitor electrode 119, the TFT 120 and the drain bus line 114 are covered with a protecting film 131, and the pixel electrode 116 is disposed on the protecting film 131. The pixel electrode 116 is made of a transparent conductive film of ITO (Indium-Tin Oxide) or the like, and is electrically connected to the source electrode 122 of the TFT 120 and the storage capacitor electrode 119 through contact holes 125 and 126 formed in the protecting film 131. Besides, the two slits 145 extending in oblique directions are formed in the pixel electrode 116 to be almost linear symmetrical with respect to the storage capacitor bus line. The surface of the pixel electrode 116 is covered with a vertically aligned film (not shown) made of, for example, polyimide.
A light-shielding film (BM), a CF resin layer and a common electrode 141 are formed on the opposite substrate disposed to be opposite to the TFT substrate 102. The plural bank-shaped linear projections 142 bent above the gate bus line 112 and the storage capacitor bus line 118 are formed on the common electrode 141. The linear projections 142 are arranged to be shifted from the slits 145 of the pixel electrode 116 by a half pitch and in parallel therewith.
In the MVA type liquid crystal display device as stated above, when a specified voltage is applied between the pixel electrode 116 and the common electrode 141, as shown in FIG. 18 and FIG. 20, four alignment areas α, β, γ and δ are formed in which alignment directions of liquid crystal molecules 108 are different from each other. The alignment areas α to δ are divided while the linear projection 142 and the slit 145 are made boundaries. When the linear projection 142 and the slit 145 are formed so that the areas of the alignment areas α to δ become almost equal to each other in one pixel, the direction dependency of the viewing angle characteristic of the liquid crystal display device becomes low.
In the conventional MVA type liquid crystal display device, there occurs a phenomenon in which when a screen is viewed from an oblique direction, it becomes whitish. FIG. 21 is a graph showing transmissivity characteristics (T-V characteristics) with respect to applied voltage in the conventional MVA type liquid crystal display device. The horizontal axis indicates the applied voltage (V) to the liquid crystal layer, and the vertical axis indicates the light transmissivity. A curved line L indicates a T-V characteristic in a direction (hereinafter referred to as a front direction) perpendicular to a display screen, and a curved line M indicates a T-V characteristic in a direction (hereinafter referred to as an oblique direction) in which an azimuth angle is 90° with respect to the display screen and a polar angle is 60°. Here, the azimuth angle is an angle measured in a counterclockwise direction with respect to the right direction of the display screen. The polar angle is an angle formed relative to a perpendicular line standing at the center of the display screen.
As shown in FIG. 21, when a relatively high voltage of about 3 V or higher is applied to the liquid crystal layer, the transmissivity in the front direction is higher than the transmissivity in the oblique direction. On the other hand, when a voltage of about 2 to 3 V slightly higher than a threshold voltage is applied (region surrounded by a circle), the transmissivity in the oblique direction becomes higher than the transmissivity in the front direction. As a result, in the case where the display screen is viewed from an oblique direction, a brightness difference in an effective drive voltage range becomes small. This phenomenon appears most remarkably in the change of color. That is, since the brightness difference of the three primary colors of R, G and B becomes small, when viewed from the oblique direction, there occurs a phenomenon in which the color of the whole screen becomes whitish, and the reproducibility of the colors is lowered. This phenomenon is called discolor. The discolor occurs not only in the MVA type liquid crystal display device but also in the TN mode liquid crystal display device.
Patent document 1 (U.S. Pat. No. 4,840,460) proposes that one pixel is divided into plural sub-pixels, and those sub-pixels are capacity coupled. In such a liquid crystal display device, since a potential is divided based on the capacitance ratio of the respective sub-pixels, voltages different from each other can be applied to the liquid crystals of the respective sub-pixels. Accordingly, apparently, plural regions different in the threshold of the T-V characteristic exist in one pixel. As stated above, when the plural regions different in the threshold of the T-V characteristic exist in one pixel, the phenomenon in which the transmissivity in the oblique direction becomes higher than the transmissivity in the front direction, as shown in the circle of FIG. 21, is suppressed, and as a result, the phenomenon in which the screen becomes whitish is also suppressed. As stated above, a method in which one pixel is divided into plural capacity-coupled sub-pixels to improve the display characteristic is called a capacitive coupling HT (half tone gray scale) method.
Patent document 2 (JP-A-5-66412) discloses a liquid crystal display device having a structure in which as shown in FIG. 22, a pixel electrode is divided into four sub-pixel electrodes 116a to 116d, and control capacitance electrodes 117a to 117d are disposed below the respective sub-pixel electrodes 116a to 116d through an insulating film. In this liquid crystal display device, the sizes of the control capacitance electrodes 117a to 117d are different from each other, and display voltage is applied to the control capacitance electrodes 117a to 117d through a TFT 120. Besides, in order to prevent light from leaking between the sub-pixel electrodes 116a to 116d, a control capacitance electrode 115 is disposed also between the sub-pixel electrodes 116a to 116d. 
Patent document 3 (JP-A-6-332009) also discloses a liquid crystal display device in which one pixel is divided into plural sub-pixels. In this liquid crystal display device, for example, a rubbing processing condition is changed for each sub-pixel, and pre-tilt angles of liquid crystal molecules of the sub-pixels are made different from each other.
All of these conventional techniques relate to the TN mode liquid crystal display device.
FIG. 23 shows a structure of one pixel of a conventional MVA type liquid crystal display device using the capacitive coupling HT method. FIG. 24 shows a sectional structure of the liquid crystal display device cut along line Y-Y of FIG. 23. As shown in FIG. 23 and FIG. 24, a TFT substrate 102 includes plural gate bus lines 112 formed on a glass substrate 110, and plural drain bus lines 114 crossing the gate bus lines 112 through an insulating film 130. The pitch of the gate bus lines 112 is, for example, about 300 μm, and the pitch of the drain bus lines 114 is, for example, about 100 μm. Rectangular pixel areas are defined by the gate bus lines 112 and the drain bus lines 114. Besides, on the TFT substrate 102, a storage capacitor bus line 118 is formed to be arranged in parallel with the gate bus line 112 and to cross the center of each of the pixel areas.
A TFT 120, control capacitance electrodes 133 and 134, and pixel electrodes 116a to 116d are formed for each of the pixel areas on the TFT substrate 102. The pixel electrodes 116a to 116d are mutually divided by slits 145. The slits 145 are extended in oblique directions, and are formed to be almost linear symmetrical with respect to the storage capacitor bus line 118.
The TFT 120 uses a part of the gate bus line 112 as its gate electrode. A drain electrode 121 of the TFT 120 is electrically connected to the drain bus line 114. A source electrode 122 is disposed at a position opposite to the drain electrode 121 through a channel protecting film 128 formed on the gate bus line 112. Besides, the source electrode 122 is electrically connected to the control capacitance electrodes 133 and 134.
The sub-pixel electrodes 116a to 116d are made of transparent electrode films of ITO or the like, and are mutually formed in the same layer. The width of the slit 145 to separate these sub-pixel electrodes 116a to 116d is, for example, 10 μm. The sub-pixel electrode 116a is electrically connected to the control capacitance electrode 133 through a contact hole 125, and the sub-pixel electrode 116d is electrically connected to the control capacitance electrode 133 through a contact hole 127. Partial areas of the sub-pixel electrodes 116b and 116c overlap with the control capacitance electrode 133 (134) through a protecting film 131. The sub-pixel electrodes 116b and 116c are indirectly connected to the control capacitance electrodes 133 and 134 by capacitive coupling through the control capacitance formed in the area. The control capacitance electrode 134 opposite to the storage capacitor bus line 118 through the insulating film 130 functions also as one electrode of the storage capacitor formed for each pixel while the storage capacitor bus line 118 is made the other electrode. The sub-pixel electrodes 116a to 116d are covered with a vertically aligned film 150 made of, for example, polyimide.
On the other hand, BMs 148 are formed on an opposite substrate 104. The BMs 148 are made of metal material such as, for example, Cr (chromium), and are disposed at positions opposite to the gate bus line 112 on the TFT substrate 102 side, the storage capacitor bus line 118, the drain bus line 114, and the TFT 120. A CF resin layer 140 is formed on the BM 148. The CF resin layer 140 of one color of R, G and B is disposed in each of the pixels.
A common electrode 141 made of a transparent conductive film of ITO or the like is formed on the CF resin layer 140. A bank-shaped linear projection 142 as an alignment regulating structure is formed on the common electrode 141. As shown in FIG. 23, the linear projection 142 is bent above the gate bus line 112 and the storage capacitor bus line 118, and is formed to be shifted from the slit 145 of the TFT substrate 102 by a half pitch and to be arranged in parallel therewith. The surfaces of the common electrode 141 and the linear projection 142 are covered with a vertically aligned film 151 made of, for example, polyimide.
When a specified gradation voltage is applied to the drain bus line 114, and a scanning signal is supplied to the gate bus line 112, the TFT 120 is turned on. When the TFT 120 is turned on, the gradation voltage is applied to the sub-pixel electrodes 116a and 116d electrically connected to the source electrode 122 and the control capacitance electrodes 133 and 134. Besides, since the sub-pixel electrodes 116b and 116c are capacity coupled to the control capacitance electrode 133 (134), the specified voltage is applied also to the sub-pixel electrodes 116b and 116c. 
However, in the structure shown in FIG. 23 and FIG. 24, since the area of the sub-pixel electrode 116c is smaller than that of the sub-pixel electrode 116b, and an overlapping area with the control capacitance electrode 133 (134) is large, the voltage of the sub-pixel electrode 116c becomes higher than the voltage of the sub-pixel electrode 116b. When the voltage of the sub-pixel electrode 116a is A, the voltage of the sub-pixel electrode 116b is B, the voltage of the sub-pixel electrode 116c is C, and the voltage of the sub-pixel electrode 116d is D, A=D>C>B is established.
When the voltages are applied to the sub-pixel electrodes 116a to 116d as stated above, the liquid crystal molecules are inclined in the direction perpendicular to the direction in which the linear projection 142 and the slit 145 extend. At this time, the inclined directions of the liquid crystal molecules become opposite directions at both sides of each of the linear projection 142 and the slit 145. Since the different voltages are applied to the sub-pixel electrodes 116a and 116d, the sub-pixel electrode 116b and the sub-pixel electrode 116c, apparently, three areas where the thresholds of the T-V characteristics are mutually different exist in one pixel. By this, the phenomenon is suppressed in which when the screen is viewed from the oblique direction, the screen becomes whitish.
However, in the liquid crystal display device shown in FIG. 23 and FIG. 24, since the control capacitance electrodes 133 and 134 are formed of metal layers which are the same layers as the source electrode 122 and the drain electrode 121 and shield the light, there is a problem that the aperture ratio of the pixel is lowered and the brightness is lowered.
Besides, according to the film thickness of the protecting film 131 formed between the pixel electrodes 116b, 116c and the control capacitance electrodes 133, 134, light transmissivity, color viewing angle, the shift amount of a common potential and the like are degraded, and there is a problem that an excellent display quality can not be obtained.